Nonwoven sheet materials are often used as the backing or web component of tapes and the like. These tapes are commonly used in the health-care industry for affixation of a variety of articles, such as dressings and tubings, and as backings and affixation materials for pre-made dressings, such as first-aid dressings and island-type dressings. They are also commonly used as affixation materials on certain types of products, such as diagnostic electrodes, surgical grounding plates, and monitoring electrodes.
Tapes formed from nonwoven sheet materials fall into two general categories based on performance needs. Category I includes sheet materials, and tapes made therefrom, that can be torn in the cross machine or cross web direction. However, these materials often cannot be torn cleanly, and therefore, display uneven or irregular torn edges. On the other hand, category II includes those sheet materials and tapes that, for practical purposes, cannot be torn in either the machine or web direction or the cross web direction.
In general, category I nonwoven materials are predominantly comprised of cellulosic fibers, and exhibit a web direction to cross web direction tensile strength ratio of less than 2.5 to 1. Cellulosic fibers are inherently fracturable (i.e., are easily broken under stress), as opposed to many synthetic, polymeric fibers, that are essentially nonfracturable.
The cellulosic fibers used in category I sheet materials are typically bonded together by a chemical binder that immobilizes, or partially immobilizes the fibers. In addition, the chemical binder increases the density of the sheet materials, and provides other advantageous properties, such as enhanced tensile strength, elongation at break, Hand (i.e., conformability), fuzzing, and the specific tear characteristics noted above. However, these advantageous properties are rapidly compromised when the sheet material becomes wet, and especially when it becomes saturated with water or other water-based fluids.
Category II materials are most frequently formed from essentially nonfracturable synthetic fibers, and are either thermally, mechanically, or chemically bonded to provide structural integrity to the sheet materials. These materials can display enhanced tensile strength, elongation, Hand, and fuzz formation depending on their particular construction. For example, mechanically bonded category II materials are typically softer and more fuzzy, in comparison with the chemically bonded materials, that tend to be stiffer and less fuzzy. However, in virtually all instances, category II sheet materials are essentially incapable of being torn in the cross web direction, and thus, do not meet the affixation requirements of the health-care industry.
Both category I and II nonwoven sheet materials and tapes enjoy reasonably extensive use in the wound treatment and medical device affixation areas of the practice of health-care. However, neither type of material has been able to make significant advances into the broader areas of the health-care market due to their inherent limitations.
Category I materials lack water resistance, and are unable to provide sufficient strength, while still maintaining softness, Hand and/or reasonable tear characteristics. Strength can be improved by changing the web direction to cross web direction orientation ratios of the fibers at the expense of tear. In addition, strength can also be improved by increasing the basic fiber content and weight at the expense of Hand and tear.
Altering the characteristics of category II sheet materials made with synthetic polymer fibers is even more restrictive. Reasonably good tear can only be achieved by utilizing fibers that make the sheet materials, and resulting tapes, very stiff. In so doing, the fiber-to-fiber bonds are essentially locked-up, thereby reducing fabric conformability, and providing a tear which is extremely difficult, and not satisfactory in terms of ragged edges and failure to tear straight.
Many attempts have been made in recent years to enhance the characteristics of category I and II materials, or to provide nonwoven sheet materials and tapes with the desirable characteristics of both category I and II materials. In so doing, different fiber types, contents, and weights of the nonwoven sheet materials have been tried. In addition, various bonding techniques, including bonding with a chemical sizing agent, physical entanglement of the web (e.g., hydroentanglement) and thermal bonding, such as through thermal embossing, have been employed. See, e.g., U.S. Pat. No. 4,973,513 (chemical bonding with LAB), U.S. Pat. No. 4,341,213 (chemical bonding to increase strength and flammability), U.S. Pat. No. 4,772,499 (hydroentanglement and partial chemical bonding), U.S. Pat. No. 3,737,368, and U.S. Pat. No. 3,507,943 (thermal embossing with engraved rollers).
For example, U.S. Pat. No. 3,121,021 discloses surgical adhesive tape formed from a tissue backing of rayon staple fibers coated with a non-tacky hydrophobic rubbery fiber-sizing polymer. The polymer-bonded backing is coated with a thin layer of pressure-sensitive adhesive that exhibits a microporous structure after drying. Incorporation of the hydrophobic rubbery fiber-sizing polymer serves to increase the water repellency, and thus, the wet strength of this category I material. Similarly, U.S. Pat. No. 4,112,177 provides essentially the same nonwoven backing as with U.S. Pat. No. 3,121,021, however, multiple adhesive layers are applied to the backing to improve the overall adhesive properties of the tapes formed therefrom. A further example of a porous, double-coated adhesive tape is disclosed in U.S. Pat. No. 4,844,973.
U.S. Pat. No. 4,292,360 discloses a multi-ply nonwoven sheet material that can be used to make pressure-sensitive adhesive tapes. The sheet materials are comprised of two nonwoven webs that are overlaid and bonded together by a rewettable chemical binder. The nonwoven webs can be formed of any type or combination of staple fibers, either alone, or in combination with binder fibers. In addition to the chemical binder, the sheet materials can also be optionally calendered or embossed.
U.S. Pat. No. 3,908,650 discloses a microporous tape formed from a nonwoven web coated on one side with a porous layer of a pressure-sensitive adhesive, and on the other with a porous thermoplastic film. The fibers adjacent the thermoplastic layer are, at least to some extent, water repellent. Optionally, the fibrous web may be thermally bonded or chemically bonded with a sizing agent. Utilization of the thermoplastic layer imparts increased abrasion and soil resistance to the overall tape.
U.S. Pat. No. 4,772,499 discloses a nonwoven web that is readily tearable in the cross web direction. The tearability of the web is enhanced by pattern bonding portions of the web with a bonding agent. After drying, the web is stated to be readily tearable in the cross web direction along the non-bonded portions of the web. Also, U.S. Pat. No. 4,303,724 discloses the use of texturized or false twist yarns in the filing of nonwoven fabrics to improve their tear characteristics.
West German Patent No. DE 1 595 300 discloses nonwoven fabrics formed from wet-laid webs that are hot calendered while the web still retains from 10% to 40% by weight residual moisture. These webs are comprised of unstretched polyester binder fibers, and optionally can include stretched polyester fibers, polyacrylamide fibers, and/or polyamide imide fibers. Further examples of thermal bonding as the principal means of reinforcing nonwoven materials can also be found in U.S. Pat. Nos. 4,731,277, 4,639,390, 4,511,615, 4,490,427, and 4,083,913. In addition, thermal bonding can be brought about by embossing such sheet materials using heated, engraved rollers. See, e.g., U.S. Pat. Nos. 3,737,368 and 3,507,943.
U.S. Pat. No. 4,490,425 discloses a soft and fluffy nonwoven fabric formed by thermal bonding staple fibers, endless fibers, or both, and needle puncturing (i.e., tacking) one or both sides of the fabric to form the fluffy surface. Thereafter, one or more of the sides are coated with a thermal adhesive to yield a fabric useable as an interlining in various garments. Similar interlining materials and methods of their preparation are also disclosed in U.S. Pat. Nos. 4,451,314 and 4,148,958.
None of the previously described fabrics or tapes has successfully combined the advantages of category I and II materials, while eliminating their shortcomings. In fact, to date, no single nonwoven sheet material, or tape made therefrom, exhibits enhanced dry strength, comparable wet strength, and ease of tear in the cross web direction, while maintaining reasonable Hand values.